| 摘要 |
PROBLEM TO BE SOLVED: To provide a thick steel plate having excellent brittle crack arrest properties which satisfies high strength (tensile strength of≥490 MPa) and also satisfies 4,500 MPa×mm<SP>1/2</SP>in Kca value at -10°C even for the plate thickness exceeding 50 mm without causing an increase in the cost. SOLUTION: In the thick steel plate, a chemical component composition is suitably regulated, and in a microstructure at a position at a depth of t/4 to t/2 (t denotes plate thickness, and the same hereinafter) from the surface, an average area ratio of pseudo polygonal-ferrite is 30 to 85%, and when the average crystal grain sizes at positions at depths of t/4 and t/2 are defined as d(t/4) and d(t/2), respectively, K<SB>0</SB>value prescribed by prescribed expression satisfies the relationship of (K<SB>0</SB>>6,750). COPYRIGHT: (C)2010,JPO&INPIT
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